1. Field of the Invention
The present invention concerns wireless communication systems, such as CDMA wireless telephone systems, and is particularly directed to the problem of estimating the signal-to-noise ratio on the forward traffic channel of a wireless communication system.
2. Description of the Related Art
In wireless communications technology, user data (e.g., speech) modulates a radio frequency signal for transmission and reception between a base station (BS) and a mobile station (MS) or mobile unit. The radio spectrum allocated by regulatory authorities for a wireless system is “trunked” to allow simultaneous use of a spectrum block by multiple units.
A common form of trunked access is frequency-division multiple access (FDMA). In FDMA, the spectrum is divided into frequency channels comprised of distinct portions of the spectrum. The limited frequency channels are allocated to users as needed. However, once a frequency channel is assigned to a user, that frequency channel is used exclusively by the user until the user no longer needs the channel. This limits the number of concurrent users of each frequency channel to one, and the total number of users of the entire system, at any instant, to the number of available frequency channels. However, in most cases a single user generally will not at all times use the full capacity of the channel assigned to him. Accordingly, obtaining maximum efficiency of the available resources is difficult to achieve by solely using FDMA.
Another common trunking system is time-division multiple access (TDMA). TDMA is commonly used in telephone networks, especially in cellular telephone systems, in combination with a FDMA structure. In TDMA, data (e.g., speech) are digitized and compressed to eliminate redundancy, thus decreasing the average number of bits required to be transmitted and received for the same amount of information. The time line of each of the frequency channels used by the TDMA system is divided into “frames” and each of the users sharing the common channel is assigned a time slot within each of the frames. Each user then transmits or receives a burst of data during its assigned time slot and does not transmit or receive during other times. With the exception of delays required by the bursty data transmission, which typically are small enough to be largely unnoticeable, the TDMA system will appear to the users sharing the frequency channel to have provided an entire channel to each user. The FDMA and TDMA combination technique is used by the GSM (global system for mobile communications) digital cellular system.
Yet another method for sharing a common channel between multiple users is code-division multiple access (CDMA) which uses direct sequence spread spectrum modulation. As with TDMA, the CDMA systems are typically used in conjunction with a FDMA structure, although this is not required. However, unlike the TDMA system, the CDMA system generally does not separate the multiple users of a common frequency channel using time slices. Rather, in CDMA, multiple users are separated from each other by superimposing a user-specific high-speed code on the data of each user. Because the applied code has the effect of spreading the bandwidth of each user's transmission, the CDMA system commonly is called a “spread spectrum” system.
Initially, the user information is digitized so that the information is represented as a sequence of “0” and “1” bits. For modulation purposes, it is common to convert this sequence of information bits into a corresponding synchronous time signal having +1 and −1 values, where +1 corresponds to a bit value of “0” and −1 corresponds to a bit value of “1”.
“Direct sequence” spreading typically is accomplished by multiplying a narrowband information signal by a much wider band spreading signal. The error and redundancy encoded digital data (speech) for each of the shared users of the CDMA channel may, for example, be provided at a rate of 19.2 kilobits per second (kbps). These data are then spread using a much higher frequency spreading signal, which may, for example, be provided at a rate of 1.2288 megabits per second (Mbps). Using the wider frequency spreading signal, a CDMA frequency channel can accommodate many users on code sub-channels.
The spreading signal is usually a sequence of bits selected from one of 64 different orthogonal waveforms generated using Walsh functions. Specifically, each such Walsh function typically consists of a repeating 64-bit sequence. A different one of the 64 different Walsh functions is utilized for each sub-channel to be included in the frequency channel. At the receiving end, a particular sub-channel can be decoded using the same Walsh function which was used to encode the sub-channel. When decoded in this manner, the desired sub-channel signal is reproduced and the signals from the other 63 sub-channels are output as low level noise. As a result, a user can distinguish its code sub-channel from other users' sub-channels on the same frequency channel.
In addition to the above channel coding, the various sub-channels also are processed using other types of coding. For instance, data on the speech traffic sub-channels typically are encrypted using a repeating pseudo-random bit sequence (long code) which is unique to each different mobile unit and which has a period of 242−1 bits. In order to coordinate encryption codes with the base unit, upon initial registration with the base unit the mobile unit provides its serial number to the base unit. The base unit then uses that serial number to retrieve the mobile unit's unique encryption code from the base station's database. Thereafter, the two can communicate using encrypted data, so as to provide a certain amount of privacy.
In addition to encryption coding, each sub-channel typically is encoded using an additional repeating pseudo-random bit sequence (PN code). The PN code sequence, also referred to as the chipping function, utilized by a particular base station may be expressed as c(t), because it is applied as a function of time t. The PN code sequence is generated using a linear feedback shift register (LFSR) which outputs a pseudo-random sequence of digital ones and zeros. These digital ones and zeros are converted to −1 and +1 symbols respectively and then filtered to give the chipping function c(t). Thus, the chipping function has the property that c(t)2=+1. The period of the PN code sequence generated by a N-register LFSR is 2N−1 bits (or chips) long, although it is common to insert a zero to extend the full sequence length to 2N chips. Typically, the PN code is generated using a 15-bit code word and a 15-register LFSR, providing a repeating sequence of 215=32,768 chips.
Generally, the PN code is identical for all base stations in the cellular network. However, each base station typically applies the PN code using a different time delay from the other base stations. For example, each base station generally selects from one of 512 different offsets (spaced 64 chips apart) for use in its PN code. By utilizing different offsets in this manner, a mobile unit can selectively tune to any given base station merely by using the same offset as that base station. Accordingly, it can be seen that merely time shifting a PN code sequence in this manner produces the same result as if each base station were using an entirely different PN code.
In a typical system, each base station uses one of the sub-channels to broadcast a pilot signal. The pilot signal has no underlying information and usually consists of all binary zeroes (symbol =1). This pattern of all “1” symbols is modulated with the appropriate Walsh function so as to occupy sub-channel 0, and then is further coded using the base station's PN code, applied with the base station's designated delay. The main purpose of the pilot sub-channel is for use by the mobile units to synchronize themselves with the base station so the mobile units can effectively communicate with the base station.
When a mobile unit is powered on, it initially searches for a pilot signal, in an attempt to establish a lock with a base station. This process is called “acquisition”. It is again noted that the only difference among the base stations in this regard is the time delay each base station applies to its PN code sequence. Thus, in order to “acquire”, or lock on to, a base station, the mobile unit must align its locally generated version of the PN code sequence with the PN code sequence of the base station by determining the timing of the transmitted pilot's spreading sequence.
Therefore, at power up a mobile unit searches for a time delay which produces a sufficiently strong pilot signal to indicate to the mobile unit that it has acquired a base station. The acquisition process generally involves multiplying a received signal with different time-delayed versions of the PN code sequence and then identifying the version which resulted in the greatest signal strength, or identifying the first time-delayed version which results in a signal strength that exceeds a threshold level. The mobile unit then applies the identified time-delay to the PN code sequence used in its transmissions and receptions in all future communications with that base station. An acquisition process is described in co-pending U.S. patent application Ser. No. 08/956,057, titled “Method and Apparatus for Accelerated Acquisition of Base Stations by Buffering Samples”, filed Oct. 22, 1997, which application is incorporated herein by reference as though set forth herein in full.
The acquisition process thus identifies the strongest base station, or at least a sufficiently strong base station, to monitor and with which to communicate. In a similar process, after acquisition has been completed the mobile unit periodically searches for the pilot signals of neighboring base stations that might have stronger signals than the base station with which the mobile unit currently is communicating. Once such a stronger base station is found, a handoff will occur in which the mobile unit will transition to communicating with the new base station. This might be necessary, for example, if the mobile unit is physically moved away from its current base station and toward the new base station. An example of a search process is described in co-pending U.S. patent application Ser. No. 09/255,032, titled “Accelerated Base Station Searching by Buffering Samples”, filed Feb. 22, 1999, which application is incorporated herein by reference as though set forth herein in full.
In the course of signal processing by the mobile unit, it is often necessary or desirable to obtain an estimate of the signal-to-noise ratio of a traffic channel on the wireless link from the base station to the mobile unit (the “forward link”). For instance, fast forward link power control consists of an inner loop and an outer loop. In the inner loop, the mobile unit compares the estimated signal-to-noise ratio for a forward link traffic channel to a target signal-to-noise ratio and, based on that comparison, instructs the base station (via the reverse link) to increase or decrease its transmission power on the subject forward link traffic channel. In the outer loop, the target signal-to-noise ratio for the forward link traffic channel is set based on a desired frame error rate (FER) and evaluations of whether incoming frames are good or bad.
Estimation of the signal-to-noise ratio in this context has to be done quickly, e.g., within a single power control group (1.25 millisecond (ms) for IS-2000), and accurately. Similarly, fast and accurate estimation of the signal-to-noise ratio for a forward link traffic channel is desirable in other contexts in which determining quality of the incoming signal is important.